1. Field of Invention
The present invention relates to the technical field of an electro-optical device such as a liquid-crystal display device. More particularly, the present invention relates to the technical field of an electro-optical device, such as a thin-film transistor (hereinafter referred as TFT) active-matrix liquid-crystal display device, which adopts an alternating drive method in which the polarities of the voltages applied to adjacent pixel electrodes are periodically alternated for every pixel row or every pixel column, so that the voltages applied to adjacent pixel electrodes in a row direction or in a column direction are inverted in polarity.
2. Description of Related Art
Electro-optical devices, such as liquid-crystal display devices, include an electro-optical material, such as a liquid crystal, interposed between a pair of substrates. The alignment state of the electro-optical material is controlled by the property of the electro-optical material and an alignment layer formed on the substrate on its surface facing the electro-optical material. If there is a step in the surface of the alignment layer (in other words, if there is a step in the surface of the pixel electrode beneath the alignment layer or in the surface of an interlayer insulator serving a substrate for the pixel electrode), an orientation defect (a disclination) occurs in the electro-optical material, depending on the magnitude of the step. If such an orientation defect occurs, proper driving of the electro-optical material in that portion becomes difficult, and the contrast ratio of the device drops due to a visible defect in the device. Since a TFT active-matrix electro-optical device includes, on a TFT array substrate, TFTs in many locations thereof for controlling and switching a variety of lines such as scanning lines, data lines, and capacitive lines, and pixel electrodes, a step inevitably occurs in the surface of an alignment layer in accordance with the presence of the lines and elements, if no planarizing process is performed.
Conventionally, the portion of the substrate suffering from such a step is aligned with the spacing between adjacent pixel electrodes, and a light-shielding layer called a black mask or a black matrix covers the portion of the step (i.e., the spacing between the pixel electrodes) so that the portion of the electro-optical material suffering from the orientation defect may remain hidden or may not contribute to display light.
Techniques for planarizing the surface of a substrate beneath the pixel electrode has been developed, in which an interlayer insulator beneath the pixel electrode is fabricated of a planarized film, such as an organic SOG (Spin On Glass) film, so that a step resulting from the presence of lines and TFTs may not be created.
The electro-optical device of this sort typically adopts an alternating drive method in which the polarity of a potential applied to the pixel electrodes is alternated at a predetermined pattern to prevent degradation of the electro-optical material as a result of the application of a direct current voltage and to control a cross-talk and flickering of a display screen image. A 1H alternating drive method is relatively easy to control and presents a high-quality image display, wherein during the presentation of a video signal of one frame or one field, the pixel electrodes arranged on an odd row are driven by a positive polarity relative to the potential of an opposing electrode, while the pixel electrodes arranged on an even row are driven by an negative polarity relative to the potential of the opposing electrode, and during the presentation of a video signal of a next frame or a next field, conversely, the pixel electrodes arranged on the even row are driven by a positive polarity while the pixel electrodes arranged on the odd row are driven by a negative polarity (in other words, the pixel electrodes on the same row are driven by the same polarity potential and the potential polarity is alternated every row with the period of frame or field). A 1S alternating drive method is also easy to control and presents a high-quality image display, wherein the pixel electrodes on the same column are driven by the same polarity potential while the potential polarity is alternated every column with the period of frame or field.
The technique to cover the above-referenced step with the light-shielding layer narrows the aperture of the pixel depending on the size of the step portion, and cannot meet the basic requirement in the technical field of the electro-optical device that the aperture ratio of the pixel be increased in a limited image display area to present a brighter image. The number of lines and TFTs per unit area increases as the pixel pitch becomes fine for high-definition video presentation. Since there is a limitation to the miniaturization of the lines and the TFTs, the ratio of the step portion to the image display area becomes relatively high, and the problem of the step portion becomes serious as high-definition design is promoted in the electro-optical device.
In accordance with the above-referenced technique for planarizing the interlayer insulator beneath the pixel electrodes, no particular problem will be presented when adjacent pixel electrodes are of the same polarity in a TFT array substrate. When the phases of the voltages (the voltages applied to the pixel electrodes adjacent in the column direction in the 1H alternating drive method, and the voltages applied to the pixel electrodes adjacent in the row direction in the 1S alternating drive method) are opposite in polarity as in the above-referenced 1H alternating drive method or 1S alternating drive method, the gap between the pixel electrode and the opposing electrode becomes wider at the edge of the pixel electrode over the line and the TFT when the planarizing process is performed than when no planarizing process is performed. A transverse electric field taking place between the adjacent pixel electrodes (specifically, an electric field in parallel with the surface of the substrate or an slant electric field having a component in parallel with the surface of the substrate) relatively intensifies. If such a transverse electric field is applied to the electro-optical material which is expected to work under a longitudinal electric field present between the pixel electrodes and the opposing electrode (i.e., an electric field perpendicular to the surface of the substrate), an orientation defect takes place in the electro-optical material, visible defect occurs there, and the contrast ratio drops. Although the area of the transverse electric field can be covered with the light-shielding layer, the aperture of the pixel is reduced with the area of the transverse electric field. As the distance between the adjacent pixel electrodes shrinks with a fine pixel pitch, the transverse electric field intensifies, and these become more problematic as high-definition design is promoted more in the electro-optical device.
The present invention has been developed in view of at least the above problems. It is an object of the present invention to at least provide an electro-optical device such as a liquid-crystal display device, which may present a high aperture ratio of pixel while displaying a high-contrast-ratio, bright and high-quality image, by generally reducing an orientation defect resulting from a step in the surface of a substrate in contact with an electro-optical material, such as a liquid crystal, and an orientation defect resulting from a transverse electric field.
An electro-optical device of an exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes, a second substrate having an opposing electrode facing the pixel electrodes, and an electro-optical material interposed between the first substrate and the second substrate, wherein the thickness of the electro-optical material between adjacent pixel electrodes which are driven by mutually opposite polarity voltages may be set to be thinner than the thickness of the electro-optical material between adjacent pixel electrodes which are driven by the same polarity voltages.
In the electro-optical device of this exemplary embodiment of the present invention, the pixel electrodes may be driven in an alternating drive manner on a row by row basis or on a column by column basis. The thickness of the electro-optical material between pixel electrodes aligned in perpendicular to a row or a column of the pixel electrodes which is driven in an alternating drive manner may be set to be thinner than the thickness of the electro-optical material between pixel electrodes aligned with the row or the column of the pixel electrodes which is driven in an alternating drive manner.
A 1H alternating drive method and a 1S alternating drive method effectively work as the alternating drive manner.
Since the thickness of the electro-optical material between the adjacent pixel electrodes which are driven by mutually opposite polarity voltages is thin in this arrangement, a longitudinal electric field taking place between the pixel electrode and the opposing electrode is intensified. The longitudinal electric field is intensified relative to a transverse electric field in an area where the transverse electric field is generated, and the occurrence of an orientation defect of the electro-optical material due to the transverse electric field is reduced.
In the electro-optical device of this exemplary embodiment of the present invention, the first substrate may include a plurality of projections formed beneath the pixel electrodes in a position corresponding to the spacing between the adjacent pixel electrodes which are driven by the mutually opposite polarity voltages.
The second substrate may include a plurality of projections formed beneath the opposing electrode in a position corresponding to the spacing between the adjacent pixel electrodes which are driven by the mutually opposite polarity voltages.
The projections on the first substrate may be formed by laminating an insulating layer and a wiring layer on the first flat substrate.
The projections on the second substrate may form a light-shielding layer.
It is contemplated that the cross section of the projections, sectioned in a plane perpendicular to the length direction of the projections, may have a variety of shapes, such as a trapezoid, a triangle, or a semi-circle.
The projections may be produced by making use of a conductor layer or interlayer insulator, forming lines and thin-film transistors, for instance, or may be fabricated by locally adding a film for the projections between the first substrate and the pixel electrode in a lamination process.
As long as the cross-sectional shape of the projections is determined in accordance with the property of the electro-optical material such as a liquid crystal so that the orientation defect of the electro-optical material resulting from the step is minimized, the projections is considered consistent with the object of the present invention even if the projections partly increases the thickness of the electro-optical material.
The edge portion of each of the adjacent pixel electrodes may be positioned on top of the projections.
In this case, the spacing between the edges of the adjacent pixel electrodes is preferably approximately equal to the distance between the opposing electrode on the second substrate and the edge portion of the pixel electrode.
Preferably, the spacing between the edges of the adjacent pixel electrodes is greater than half a cell gap thereof.
In this embodiment, the longitudinal electric field is intensified relative to the transverse electric field to the degree that the adverse effect of the transverse electric field is not pronounced. Without thinning the thickness of the electro-optical material, the spacing between the pixel electrodes is narrowed. If the pixel pitch becomes fine, not only the aperture ratio is maintained, but also the thickness of the electro-optical material is controlled.
The thickness of the projections is preferably at least 300 nm.
With this arrangement, the longitudinal electric field in the area where the transverse electric field is generated intensifies as the thickness of the electro-optical material is reduced. Since the step is raised to be 300 nm or more in an area where groups of pixel electrodes are adjacent to each other, the thickness is reduced accordingly, and the longitudinal electric field is intensified relative to the transverse electric field in the area to the degree that the adverse effect of the transverse electric field is not pronounced in practice.
When the electro-optical material is a TN (Twisted Nematic) liquid crystal, the projections preferably includes an inclined sidewall, and the pretilt angle of the twisted nematic liquid crystal is preferably equal to the inclination angle of the inclined sidewall of the projections.
In accordance with this embodiment, in principle, the TN liquid-crystal molecules, substantially in parallel with the surface of the substrate with no voltage applied, are aligned to be gradually twisted from the first substrate to the second substrate. With the tapered sidewall formed on the substrate surface, a good liquid-crystal alignment state, nearly as good as when the thickness of the TN liquid crystal remains fixed at the center of the pixel electrode, is obtained even if the thickness of the TN liquid crystal gradually decreases as it runs laterally. Specifically, the liquid crystal orientation defect due to the step is minimized in the portion of the liquid crystal that is locally thinned to reduce the liquid-crystal orientation defect attributed to the transverse electric field.
Since the pretilt angle of the TN liquid crystal in the first substrate matches the inclination angle of the inclined sidewall of the projections in accordance with this embodiment, the TN liquid-crystal molecules, substantially in parallel with the surface of the substrate with no voltage applied, are aligned to be inclined at a pretilt angle as large as several degrees with respect to the surface of the substrate. With the pretilt angle of the TN liquid crystal on the first substrate matching the inclination angle of the tapered sidewall, a good liquid-crystal alignment state, nearly as good as when the thickness of the TN liquid crystal remains fixed at the center of the pixel electrode, is obtained even if the thickness of the TN liquid crystal gradually decreases toward as it runs laterally. Here, xe2x80x9cthe pretilt angle of the TN liquid crystal on the first substrate matching the inclination angle of the tapered sidewallxe2x80x9d means that both angles correspond to the degree that a good liquid-crystal orientation state nearly as good as when the thickness of the TN liquid crystal remains constant is obtained, and a permissible range of agreement is determined experimentally, theoretically and through experience.
The electro-optical material may be a VA (Vertically Aligned) liquid crystal, and the projections may include a sidewall substantially perpendicular to the surface of the first substrate.
In accordance with this embodiment, in principle, VA liquid-crystal molecules are aligned to be substantially perpendicular to the substrate with no voltage applied state, and the liquid crystal orientation is forced to be disturbed in an area where a border of substrate surfaces different in level is present. If the border of the substrate surfaces rises vertically, the portion of the liquid crystal subject to orientation disturbance is reduced in the area. A liquid crystal orientation state, nearly as good as when the thickness of the VA liquid crystal remains fixed, is obtained in a portion of the liquid crystal in a substantially flat area on the substrate relatively higher in level and in a portion of the liquid crystal in a flat area on the substrate relatively lower in level. The liquid crystal orientation defect resulting from the step where the thickness of the liquid crystal is locally thinned to reduce the liquid crystal orientation defect attributed to the transverse electric field is reduced.
In the electro-optical device of this exemplary embodiment of the present invention, the first substrate may include a flat area formed on the side thereof facing the electro-optical material in a position corresponding to the spacing between the adjacent pixel electrodes which are driven by the same polarity voltages.
The first substrate preferably includes a groove on the flat area of the surface thereof, and a line is preferably formed in an area corresponding to the groove.
In accordance with this embodiment, the planarizing process is relatively easily performed for a relatively high flatness by forming a groove by etching the first substrate and an interlayer insulator to be positioned beneath the lines, such as the data line and the scanning line, and by burying the data line and scanning line into the groove.
An electro-optical device of another exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes, a second substrate having an opposing electrode facing the pixel electrodes, an electro-optical material interposed between the first substrate and the second substrate, and a plurality of projections formed on the first substrate beneath the pixel electrodes in a position corresponding to the spacing between adjacent pixel electrodes which are driven by mutually opposite polarity voltages.
An electro-optical device of another exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes, a second substrate having an opposing electrode facing the pixel electrodes, an electro-optical material interposed between the first substrate and the second substrate, and a plurality of projections formed on the second substrate beneath the opposing electrode in a position corresponding to the spacing between adjacent pixel electrodes which are driven by mutually opposite polarity voltages.
An electro-optical device of another exemplary embodiment of the present invention includes a first substrate having a plurality of pixel electrodes, a second substrate having an opposing electrode facing the pixel electrodes, an electro-optical material interposed between the first substrate and the second substrate, and a flat area formed on the side of the first substrate facing the electro-optical material in a position corresponding to the spacing between adjacent pixel electrodes which are driven by the same polarity voltages.
An electro-optical device of another exemplary embodiment of the present invention includes an element-array substrate having a plurality of data lines, a plurality of scanning lines intersecting the data lines, a plurality of pixel electrodes arranged in a matrix, each pixel electrode arranged in an area surrounded by the data lines and the scanning lines, and a switching element, connected to the data line and the scanning line, for outputting a video signal to the pixel electrode, an opposing substrate having an opposing electrode facing the pixel electrodes, an electro-optical material interposed between the element-array substrate and the opposing substrate, a flat area formed on the element-array substrate on the side thereof facing the electro-optical material in an area along the data line, and a projection formed on the element-array substrate on the side thereof facing the electro-optical material in an area along the scanning line.
The plurality of the pixel electrodes arranged in a matrix are preferably driven in an alternating drive manner on a scanning line by scanning line basis.
The projection may be formed in an area of a capacitive line extending along the scanning line.
The projection may have a flat portion on the peak thereof.
The flat area may produced by forming a groove in an area on the element-array substrate along the data line.
An electro-optical device of another exemplary embodiment of the present invention includes an element-array substrate having a plurality of data lines, a plurality of scanning lines intersecting the data lines, a plurality of pixel electrodes arranged in a matrix, each pixel electrode arranged in an area surrounded by the data lines and the scanning lines, and a switching element, connected to the data line and the scanning line, for outputting a video signal to the pixel electrode, an opposing substrate having an opposing electrode facing the pixel electrodes, an electro-optical material interposed between the element-array substrate and the opposing substrate, a projection formed on the element-array substrate on the side thereof facing the electro-optical material in an area along the data line, and a flat area formed on the element-array substrate on the side thereof facing the electro-optical material in an area along the scanning line.
The plurality of the pixel electrodes arranged in a matrix are preferably driven in an alternating drive manner on a data line by data line basis.
The flat area may be formed in an area of a capacitive line extending along the scanning line.
The projection may have a flat portion on the peak thereof.
The flat area may produced on the element-array substrate by forming a groove in an area along the scanning line and the capacitive line.
An electro-optical device of another exemplary embodiment of the present invention includes an element-array substrate having a plurality of data lines, a plurality of scanning lines intersecting the data lines, a plurality of pixel electrodes arranged in a matrix, each pixel electrode arranged in an area surrounded by the data lines and the scanning lines, and a switching element, connected to the data line and the scanning line, for outputting a video signal to the pixel electrode, an opposing substrate having an opposing electrode facing the pixel electrodes, an electro-optical material interposed between the element-array substrate and the opposing substrate, a flat area formed on the opposing substrate on the side thereof facing the electro-optical material in an area corresponding to the data line of the element-array substrate, and a projection formed on the opposing substrate on the side thereof facing the electro-optical material in an area corresponding to the scanning line of the element-array substrate.
The plurality of the pixel electrodes arranged in a matrix are preferably driven in an alternating drive manner on a scanning line by scanning line basis.
The projection may be formed in an area of a capacitive line extending along the scanning line.
The element-array substrate may include a groove, corresponding to an area in which the data line extends, for planarizing the surface of the element-array substrate facing the electro-optical material.
The element-array substrate may include, on the surface thereof, a groove in an area corresponding to the area where the scanning line extends, for planarizing the surface of the element-array substrate facing the electro-optical material.
An electro-optical device of another exemplary embodiment of the present invention includes an element-array substrate having a plurality of data lines, a plurality of scanning lines intersecting the data lines, a plurality of pixel electrodes arranged in a matrix, each pixel electrode arranged in an area surrounded by the data lines and the scanning lines, and a switching element, connected to the data line and the scanning line, for outputting a video signal to the pixel electrode, an opposing substrate having an opposing electrode facing the pixel electrodes, an electro-optical material interposed between the element-array substrate and the opposing substrate, a projection formed on the opposing substrate on the side thereof facing the electro-optical material in an area corresponding to the data line of the element-array substrate, and a flat area formed on the opposing substrate on the side thereof facing the electro-optical material in an area corresponding to the scanning line of the element-array substrate.
The plurality of the pixel electrodes arranged in a matrix are preferably driven in an alternating drive manner on a scanning line by scanning line basis.
The element-array substrate may include, on the surface thereof, a groove, in an area corresponding to the area where which the data line extends, for planarizing the surface of the element-array substrate facing the electro-optical material.
The element-array substrate may include, on the surface thereof, a groove, in an area corresponding to the area where the scanning line extends, for planarizing the surface of the element-array substrate facing the electro-optical material.
The electro-optical device of the present invention at least reduces the orientation defect of the electro-optical material resulting from the transverse electric field and the orientation defect of the electro-optical material resulting from the step and the light-shielding layer for covering the orientation defect portions of the electro-optical material is reduced in size. The aperture ratio of each pixel is increased without creating image defects such as visible defect, and a high-contrast ratio, bright, and high-quality image is presented.